1. Field of the Invention
The present invention relates to a method and related communications device for improving discontinuous reception (DRX) functionality in a wireless communications system, and more particularly, to a method and related communications device for improving start mechanism of a DRX Inactivity Timer of the DRX functionality in a wireless communications system.
2. Description of the Prior Art
The third generation mobile telecommunications system (called 3G system) provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission, and also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates.
Long Term Evolution wireless communications system (LTE system), an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
Targeting on the present LTE system, Discontinuous Reception (DRX) functionality is applied to the MAC layer for allowing a user equipment (UE) to enter a standby mode during certain periods of time and stopping monitoring a Physical down Link Control Channel (PDCCH), so as to reduce power consumption of the UE.
In cases of using the DRX functionality, whenever a new DRX cycle begins, an On Duration Timer is started for awaking the UE to monitor the PDCCH until the On Duration Timer expires. In addition, when a DRX Inactivity Timer or a DRX Retransmission Timer is started, the UE also monitors the PDCCH.
During the time when the UE monitors the PDCCH, if the PDCCH indicates the UE to receive a packet on a Downlink Share Channel (DL-SCH) but the packet cannot be decoded successfully, the MAC layer of the UE would perform a Hybrid Automatic Repeat Request (HARQ) process to request a retransmission of the packet. Since the UE would not receive any retransmission packets during a round trip time of the HARQ process, a HARQ RTT Timer is thus configured by the DRX functionality for allowing the UE to go back to monitor the PDCCH after this period of time where the UE enters the standby mode, so that the power consumption of the UE can further be reduced.
When the HARQ RTT Timer expires, the DRX Retransmission Timer is started by the DRX functionality for providing the UE a certain amount of time to monitor the PDCCH for detecting retransmission packets of the HARQ process. If any retransmission packet of the HARQ process is detected, the DRX Retransmission Timer is stopped. Otherwise, the DRX Retransmission Timer is kept running until expiring.
On the other hand, when the On Duration Timer or the DRX Inactivity Timer is started, if the PDCCH indicates the UE to receive a new transmission packet, the DRX Inactivity Timer is then started or restarted for allowing the UE to continue monitoring the PDCCH, so as to prevent missing reception of following transmissions.
However, according to the above operation, when the DRX Retransmission Timer is running but both of the On Duration Timer and the DRX Inactivity Timer are not running, if the PDCCH indicates the UE to receive a new transmission packet, the DRX Inactivity Timer would not be started or restarted by the DRX functionality. In such a case, the UE may miss reception of the following new transmission packets due to inability of continuously mentoring the PDCCH.